If I am understanding the 259B manual correctly the "loss" is actually
"return loss" and measures the ratio of energy sent to reflected energy
received in dB. This means different things is different circumstances.
If what you are talking about is measuring the loss of coax from transmitter
to antenna, eg, power at transmitter is 1200 watts and power at antenna is
600 watts, therefore loss is 3 db in the coax (even that is a bit of a
simplification), you will need to measure it at a point that is a multiple
of an electrical half wavelength from an open ended feedline (best) or odd
multiple of a electrical quarterwave from a shorted end.
Obviously you can't just pick the frequency(ies). That has already been
decided by the velocity factor of the coax, and the length you cut it to.
Set the MFJ to read loss. Starting at a very low frequency with the far end
open, advance the frequency slowly while watching the loss figure. The loss
should exibit points of least loss and most loss as the frequency advances.
If the MFJ is measuring return loss, then the one way loss of the coax at a
minimum loss frequency is 1/2 the loss shown *for that frequency* when the
coax is well terminated in it's characteristic.
My AIM 4170 does the divide by two and also scans to a graph, making the
1/2 wave multiples obvious on a plot that looks a lot like a sine wave as
frequency advances.
I'm not at all sure whether the MFJ is doing the divide by two or not. I
didn't find that in the manual.
The loss per 100 feet at 10 MHz figure is created by an extrapolation of the
loss figures measured at the frequencies using a return loss, or it is
measured with power on the line and a meter on both ends. You can do it
with a ruler on a piece of graph paper that has a logarithmic scale for the
horizontal. This becomes very unspecific if the coax has soaked water into
the shield at some point, or has manufacturing anomalies, been crushed, etc,
etc. Interpreting a return loss vs frequency graph on a bad piece of coax is
a formula for a headache, but can be used to narrow the location of a fault.
My 259B ain't working so I can't compare it to the known AIM4170 to see
whether it's divide by 2 or not, or accurate.
73, Guy.
----- Original Message -----
From: "Gary Smith" <Gary@doctorgary.net>
To: <antennaware@contesting.com>
Sent: Thursday, December 18, 2008 8:21 PM
Subject: [Antennaware] MFJ-259B question
> Hi,
>
> I'm guessing someone here has a 259B. I bought a used one that was
> damaged in shipping. I sent it to MFJ for repair and it came back
> with continued issues, one of which was when I would have it tell me
> the coax loss of a 350' length of RG-213, the result was 12dB. I knew
> that was wrong so I tried it into a bird dummy load with a 4' length
> of coax and again the result was 12dB. When attached to an antenna,
> the farther from resonance, the lower the claimed dB.
>
> I returned it to MFJ citing this example as an obvious error. They
> repaired it and I just got it back today.
>
> I have the exact same results. If the center conductor is inserted, I
> see a small dB value but when the shield makes contact with the
> 259B, it behaves as above.
>
> If there is no coax attached, it reads 0 dB.
>
> To offer exact numbers, I just attached my 350' of RG-213 connected
> to a 160M inverted L and:
>
> @ 1.825MHz SWR is 1:1, R=53 X=5
>
> Push the mode button once and it goes to Coax Loss and it shows 12dB
>
> If using the same antenna & coax, I adjust the tune control on the
> 259B to evaluate 2.373MHz
> the SWR shows 7.3 R=17 X=66 and the coax loss shows 1.1dB
>
> I am at a complete loss as to what is going on, Based on 350' of RG-
> 213 and using this calculator http://www.timesmicrowave.com/cgi-
> bin/calculate.pl I expected to see around .9dB loss not 12dB
>
> So either I am missing the point of how to read the loss or this unit
> is still incorrectly reading the line loss. If that's the case, I
> can't feel secure about anything it's revealing. I really want to
> know the radiation resistance at the antenna to know if I am seeing a
> significant difference with additional radials. I just don't trust
> this unit.
>
> Is there something I am doing incorrectly? Or is this still defective
> after two visits to MFJ over the last three weeks?
>
> Thanks,
>
> Gary
> KA1J
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>
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